Efficiency, for an electrical system, can refer not only to minimizing losses in individual system components, but can also entail minimizing the amount of hardware in the system as well as optimizing the manner in which power is generated, converted and distributed by that electrical system. This is especially true for aircraft which needs to rely on a limited number of power generating sources and distribution components it can reasonably carry on board for providing electrical power.
Generators on aircraft may be driven with various prime movers, such as turbine engines. In many cases, a prime mover may drive a generator only as an ancillary function. A typical primary function for a prime mover, such as a turbine engine, may be to provide propulsion thrust for the aircraft. In the context of its primary function, the prime mover may operate at varying rotational speeds. Therefore, a generator coupled to a shaft of such a variable-speed prime mover may rotate at varying speeds. Additionally, a generator load may become large enough to negatively affect engine thrust output. Because of recent increases in electrical power demand of aircraft, a single generator driven by a single prime mover may not be capable of producing all of the electrical power for an aircraft. Consequently, an aircraft may be provided with multiple generators, each driven by different prime movers.
Certain aircraft operating conditions may arise in which a particular generator may be subjected to a particularly high load demand during a time when its associated prime mover may be performing its primary function (e.g. producing thrust) at a relatively low speed. In order to meet the high electrical power requirement of an attached generator, it may be necessary to increase the speed of the prime mover, even though such an increase in speed may not otherwise be required for the primary function of the prime mover. Excessive fuel may be consumed if and when a prime mover is operated at a speed greater than required for its primary role. Certain design efforts have been directed to this issue. For example multiple generators may be driven on different shafts of a turbine machine. The turbine machine may have a low-pressure turbine output shaft and a high-pressure turbine output shaft. A separate generator may be driven by each of the shafts. Electrical outputs of the generators may be shared and controlled so that electrical loads may be allocated to either the low-pressure turbine or the high-pressure turbine as a function of turbine operating speed. This allocation may facilitate more efficient operation of the turbine machine.
Historically, multiple generators and distribution channels are employed within an aircraft such that a fault in any one system will not either jeopardize safe operation of the aircraft or compromise the aircraft's ability to complete its mission. While this has proven to be a robust approach, it does create a redundancy that reduces the overall efficiency of the electrical system itself, as well as the efficiency of other aircraft systems with which it interfaces such as propulsion and thermal management systems.